Mechanical and electrical properties of a MWNT/epoxy composite

Composites Science and Technology 62 (2002) 1993–1998 www.elsevier.com/locate/compscitech

Mechanical and electrical properties of a MWNT/epoxy composite A. Allaouia, S. Baia,b, H.M. Chengb, J.B. Baia,* a

Laboratory of Mechanics of Soils, Structures and Materials, CNRS UMR 8579, Ecole Centrale Paris, 92295 Chaˆtenay-Malabry, France Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, 72 Wenhua Road, Shenyang 110016, PR China

b

Received 26 March 2002; received in revised form 7 July 2002; accepted 8 July 2002

Abstract Multi-walled carbon nanotube/epoxy resin composites have been fabricated. By choosing an over-aged hardener, relatively soft and ductile matrix, a rubbery epoxy resin, has been obtained. This made possible to evaluate the effect of nanotube addition on the whole stress-strain curve up to high strain level. The mechanical and electrical properties of the composite with different weight percentages of nanotubes have been investigated. The Young’s modulus and the yield strength have been doubled and quadrupled for composites with respectively 1 and 4 wt.% nanotubes, compared to the pure resin matrix samples. Conductivity measurements on the composite samples showed that the insulator-to-conductor transition took place for nanotube concentration between 0.5% and 1 wt.%. # 2002 Published by Elsevier Science Ltd. Keywords: B. Mechanical properties; B. Electrical properties; Multi-walled carbon nanotubes

1. Introduction Carbon nanotubes (CNTs), one graphene layer (SWNT) or many graphene layers wrapped onto themselves (MWNT) are a novel crystalline carbon form. The recent theoretical and experimental investigations indicate that they have properties suitable for applications in many fields. Their interesting mechanical (axial Young modulus 1–5 TPa [1–7], high flexibility [8], bending fully reversible up to a 110 critical angle for SWNT [9]), and physical (metallic or semi-conducting character [10,11], field emission, high thermal and electrical conductivity, hydrogen adsorption, . . .) properties have been the subject of many research works. The CNT-based composites, which may be one of the most promising applications, have been intensively studied using different matrix materials, polymers [12–20], ceramics [21–23] and metals [24,25]. A review paper has been published on the subject [26]. In the case of polymer matrix, most papers dealt with relatively brittle and rigid matrix, such as cured epoxy resin. And often, only mechanical properties [15, 17] or physical properties * Corresponding author. Tel.: +33-1411-31316; fax: +33-141131430. E-mail address: [email protected] (J.B. Bai).

[18,20] were treated. In this paper, a nanocomposite was prepared using an over-aged hardener so that the epoxy matrix remains relatively soft and ductile even after polymerization. Both mechanical and electrical properties were investigated to evaluate the change introduced by the CNTs at different weight percentages (up to 4%). With the ductile matrix, it is possible to evaluate the influence of CNTs addition on the whole stress-strain behavior, not only limited to the Young’s modulus.

2. Experimental 2.1. Materials 2.1.1. Matrix Epoxy polymer matrix was prepared by mixing 15 parts by volume of epoxy resin (Bisphenol A-epichlorhydrine) with 2 parts of aromatic hardener (triethylenetetramine). Epoxy resin contains one or more epoxide groups that serve as cross-linking points when the resin reacts with the hardener to form long chains, the polymerization. The over-aged hardener used in this work produced a rubbery epoxy matrix. The hardener has an impact on the matrix structure and the crosslinking ratio and by this way the molecular motions.

0266-3538/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S0266-3538(02)00129-X

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A. Allaoui et al. / Composites Science and Technology 62 (2002) 1993–1998

2.1.2. MWNT 2.1.2.1. Synthesis. CNTs used in this study were synthesized by (CVD) thermal decomposition of hydrocarbon gas. Benzene was used as carbon source, thiophene as growth promoter, ferrocene as catalyst and hydrogen as carrier gas. The relative ratio of different components in the reaction system was controlled by adjusting the carrier gas flow rate. Through controlling the reaction time and relative components of benzene, thiophene and ferrocene, carbon nanotubes and carbon nanofibers of different diameter and structure can be obtained. The details of our CVD system will be published elsewhere [27]. 2.1.2.2. CNTs morphology. TEM observations were carried out with a JEOL JEM 1200 EX operating at 120 kV to examine the morphology of the CVD products. The CNTs are MWNTs, with diameters in the range of 15–400 nm, the mean diameter being 100 nm, and average length a few hundreds microns (Fig. 1). They are highly entangled and randomly organized. There are some catalyst particles, amorphous carbon and onions as impurities. 2.1.2.3. Raman spectroscopy. Raman spectra were recorded with a DILOR LABRAM multi-channel confocal microspectrometer in backscattering mode using an Ar+ laser excitation (514.5nm, 5mW; resolution 1cm1). The integration time was 100s and the spectra were averaged over 2 accumulations. The incident laser beam was focused onto the specimen surface through a 100 objective lens forming a laser spot of approximately

2mm in diameter. The Raman spectra, obtained in the range 1000–2000 cm1, shows a band at 1588 cm1 (G band) due to the A(g1), E1(g) and E2(g) vibrational modes, and a band at 1357 cm1 (D band) arising from the disorder-induced A(g1) mode (Fig. 2). The G band is wide and its intensity is lower than twice that of the D band, which is typical of MWNT Raman spectra. The width of the G band is related to the CNTs size distribution whereas the intensity of the D band decreases with the degree of graphitization of the tubes. In this work, Raman spectroscopy was mainly used to verify the multiwalled nature of the CNTs and the overall homogeneity of the sample. No meaningful differences were observed at different Raman measurements points. 2.1.3. Composite preparation The as-prepared CNTs material consists of aggregates of different sizes. The bigger ones are millimetric or even centimetric. This would be an obstacle to the uniform dispersion of MWNT into the epoxy matrix. A procedure of two steps was followed. The MWNT were first dispersed in methanol solution under magnetic agitation to reduce the maximum size of the aggregates to about 100 mm. After complete evaporation of methanol, the obtained MWNT powder was then directly added to the epoxy resin Bisphenol A/aromatic hardener mixture. Finally, it was injected into sample moulds after manual homogenization. The 4 wt.% samples were very viscous and the homogenization process was difficult. The samples were placed between two metal plates under pressure to reduce porosity forming during hardening. Before

Fig. 1. Transmission electronic micrographs of CNTs.

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Fig. 2. Raman spectrum of the CVD MWNT.

mechanical and electrical measurements, the surfaces of the specimens were mechanically polished to minimize the influence of surface flaws, mainly the porosity. 2.1.4. Tensile tests and AC measurements procedure Tensile tests of dog-bone shaped specimens (Fig. 3) were performed at 25 C and at a constant cross-head rate of 1 mm/min on an Instron electro-mechanical testing machine. The choice of this quite low loading rate is to compare later with the results of more brittle and more rigid materials. The strain was measured with an extensometer. The gauge length is 12.5 mm +/2.5 mm. The thickness of samples after polishing is between 2 and 3 mm. The AC spectroscopy measurements have been performed with a Solartron 1260 Impedance/Gain Phase Analyzer. A 1 VAC voltage, of known amplitude and phase, was applied to a circuit equivalent to a resistor in parallel with a capacitor and the output current was measured. The complex impedance, Z, was consequently calculated by the analyzer.

composite were conducted to break of the samples. The stress–strain curves are shown in Fig. 4. The resin obtained is very ductile. The maximum stress levels reached are not those of a standard resin (generally 30–50 MPa). The general tendency is that the stress level is increased by the addition of CNTs which play the role of reinforcement. The Young’s modulus and the yield strength of the composite are doubled by adding 1 wt.% of CNT and quadrupled with 4 wt.% CNT. Moreover, two curves for the 1 wt.% composite are presented and show reproducible results. The mechanical properties of the composites are given in Table 1. To have a better view of the effect of CNTs on the mechanical behavior of the composites, the normalized stress-strain curves are drawn in Fig. 5. In this figure the stress of composites of different CNTs wt.% is divided by the stress of the pure matrix at the same strain level. An almost constant reinforcement coefficient has been observed for the strain level higher than 2%. The ratio is about 2 for 1 wt.% CNTs composite and 2.5 for 4 wt.% composite. It can be concluded that the reinforcement role is much reduced in the case of 4 wt.% composite. The same phenomenon was observed for Young’s modulus. The Young’s modulus of the composites normalized by that of the pure matrix is presented in Fig. 6. A slope decrease can be observed for 4 wt.% composite The optical microscope observations revealed the presence of porosities and CNTs aggregates (small size for the 1 wt.% composites and large for the 4 wt.% samples). On the polished surface of samples, there are some zones with very high local

3. Results and discussion 3.1. Mechanical properties The most tensile tests were performed to the limit strain of extensometer, 25% while those of 4 wt.%

Fig. 4. Tensile test stress–strain curves of the resin and its composites. Table 1 Young’s modulus and stresses at different strain levels

Fig. 3. Tensile test sample geometry.

CNTs wt.%

Young’s modulus MPa

Yield strength 0.2%MPa

10%Mpa

0 1 4

E0=118 236(2*E0) 465(3.9*E0)

1 3 6

4 8 10

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other hand, the porosity may be also another origin of lower reinforcement effect of CNTs in the case of 4 wt.% samples compared to the 1 wt.% samples. 3.2. Electrical properties

Fig. 5. Normalized stress–strain curves of the composites.

Fig. 6. Normalized Young’s modulus of the composites.

CNT concentrations, as shown in Fig. 7 a. The distribution of CNTs is more homogeneous in the 1 wt.% samples (Fig. 7b). This could explain partially the premature fracture occurred in 4 wt.% composites. On the

The conductivity was investigated first using a simple circuit. A direct voltage U=I V was applied directly on the sample. The intensity I through the sample thickness e and contact area S was measured, using a picoamperemeter. The DC conductivity was then calculated using the following formula: ¼ UI Se DC. The AC impedance spectroscopy gives much reliable results and allows studying the behavior at different frequencies. The sample was disposed between two parallel electrodes, constituting, hence, a capacitor during AC measurement. It was submitted to a sinusoidal perturbation of intensity 1 V with variable frequency. The analyzer detected the current (intensity and phase) through the sample at each frequency. Complex impedance is calculated as a function of frequency. The AC conductivity is given by the following relation: ¼ Y0 Se AC, Y0 is the real part of the admittance (Y=1/Z). To improve the electrical properties, MWNT were used, because those Russian doll like nanotubes are always conductive. This is not the case for SWNTs, which can be semi-conductive or metallic as a function of their chirality. The MWNT conductivity results from the mean behaviour of the different rolled graphene layers. The DC and AC electrical conductivity of MWNT/epoxy composites are plotted as a function of the weight percentage of nanotubes added to the matrix (Fig. 8). We can observe that the AC and DC results are in good agreement. Over the range of CNTs addition studied, up to 4 wt.%, nine orders of magnitude change in the conductivity was observed, corresponding to a percolation phenomenon. The AC and DC measurements

Fig. 7. (a) Optical micrographs of the surface of the 4 wt.% CNT composite. The porosities are marked by white arrows and the zones of very high local CNT concentration by black arrows. (b) Optical transmission light micrograph of the surface of 1 wt.% CNT composite.


Fig. 8. AC and DC conductivity as a function of CNT weight percentage at 100 Hz.

revealed the presence of a conduction threshold between 0.5 and 1 wt.%, a percolation threshold due to the conductive path of interconnected nanotubes. The high aspect ratio of CNTs allows reaching this threshold with a small CNT fraction. The conductivity was calculated at different frequencies (from 101 to 106 Hz). A plot of the AC conductivity as a function of the frequency is shown in Fig. 9. Two behaviors appeared: the insulators with conductivity inversely proportional to the frequency and the conductors with conductivity independent of the frequency. The 0.5 wt.% composite behavior is very similar to that of the matrix, except that its conductivity is an order of magnitude higher. With that weight percentage, the composite is still insulator. The 1 and 4 wt.% composites are conductor with conductivity as high as respectively 103 and 6102 S/cm. The addition of 1 wt.% of CNTs into the matrix is sufficient to obtain a composite suitable for applications requiring electrostatic discharge. From the electrical properties measurements, the threshold of a conducting or interconnected network of the CNTs was reached at 1 wt.%. In putting 4 wt.% CNTs, the conductivity was improved only an order of magnitude compared to 1

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wt.% samples. Same phenomenon was noticed on the mechanical properties. There is some kind ‘‘saturation effect’’. The results of this study suggest that it would not be helpful to use high CNTs concentrations to improve the mechanical properties of composites if they are in random distribution. The upper limit is governed by the distribution and the morphology (aspect ratio) of the CNTs. The higher the alignment degree of CNTs, the higher this value. The lower limit of the CNTs concentration can be determined easily by conductivity measurement. The full domain of reinforcement and/or improvement of the mechanical and electrical properties is boarded by these two limit values. This kind of fullcourage models have been proposed in the case of ductile-to-brittle fracture transition in the metal matrix composites [28]. The CNTs, like conventional carbon fibers, carbon black and graphite can reinforce the polymer matrix to form advanced nanocomposites. Qian et al. [17] have reported that by adding 1% of nanotubes into polystyrene matrices resulted in increasing of ever-all tensile modulus and strength by approximately 42 and 25%, respectively. Kymakis et al. [29] presented a percolation threshold of approximately 11 wt.% SWNTs in a P3OT polymer. As the nanotubes concentration increases from 0 to 20 wt.%, the conductivity of the resulting films increases by five orders of magnitude. Ezquerra et al. [30] concluded that lower percolation thresholds and higher conductivity values are observed upon comparing carbon black (at 1.1 wt.%) and graphite (at 12 wt.%) composites due to the differences in particle sizes. A very different picture emerges when studying carbon fibers composites, the high fiber orientation gives rise to materials with higher electric conductivity levels than those found for particulate composites. Considering the fact that the MWNTs in our work are in random orientation and often in agglomeration (Fig. 7b), good results have been obtained, both on the mechanical reinforcement effect and on the electric conductivity improvement. The preparation is simple, easy and at low cost. The further work should be aimed at improving the dispersion and the alignment of the CNTs in the matrix and the interfacial bonding between the CNTs and the matrix.

4. Conclusion

Fig. 9. AC conductivity as a function of the frequency.

This study has demonstrated the intrinsic potential of the CNT. Small quantity addition can modify considerably the mechanical and electrical behavior of a soft polymer matrix. The exceptional mechanical properties, the high aspect ratio and the good conductivity of MWNT have been used to improve the performance of a rubbery epoxy matrix nanocomposites. Considerable enhancement has been obtained with CVD MWNT and

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simple process. The addition of 1 and 4 wt.% of CNT into the epoxy matrix has a remarkable effect on the mechanical properties. The Young’s modulus and the yield strength of the 1 wt.% composite have been increased by respectively 100 and 200% compared to the pure matrix. On the other hand, the CNTs confer conductivity to the insulator matrix. An increase in the conductivity by nine orders of magnitude was observed in the range of 0–4 wt.% CNTs, as a result of a percolation phenomenon. The critical wt.% threshold was estimated to be between 0.5 and 1 wt.% CNTs. The decrease of the slope of the normalized Young’s modulus versus CNT wt.% curve between 1 and 4 wt.% has confirmed that the homogeneity of the composite is a critical point for mechanical behavior of the nanocomposites. The non-homogeneity of CNTs may have a less effect on their electrical properties. The results of this study suggest that it would not be helpful to use high CNTs concentrations to improve the mechanical properties of composites if they are in random distribution.

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